1. Field of the Invention
The present invention relates to dispersion compensating optical fibers that are suitable for use in wavelength division multiplexing (WDM) systems, and to dispersion compensating fibers that are particularly well suited for use in L-band systems that operate at wavelengths longer than 1565 nm. It also relates to dispersion compensated links utilizing such dispersion compensating fibers, and to a process for making the dispersion compensating fibers.
2. Technical Background
Telecommunications systems presently in place include single-mode optical fibers which exhibit zero dispersion at a wavelength around 1300 nm; such fibers are referred to herein as xe2x80x9cSMF fibersxe2x80x9d. Signals transmitted within such systems at wavelengths around 1300 nm remain relatively undistorted. Signals can be transmitted over such systems at wavelengths around 1550 nm in order to achieve lower loss and to utilize the effective and reliable erbium fiber amplifiers that operate in the 1550 nm window.
Over the past few years telecommunications systems have been upgraded from 2.5 Gbs single channel systems to 10 Gbs WDM systems. The increased bit rate per channel has made these systems dispersion limited. Transmission at 1550 nm over SMF fibers introduces a dispersion of about +17 ps/nmxc2x7km; such fibers are therefore restricted to about 60 kms uninterrupted transmission at 10 Gbs. The solution put forth to counter this has been to dispersion compensate at regular intervals. For example, a single-mode fiber with a dispersion of +17 ps/nmxc2x7km at 1550 nm requires a dispersion compensation of 1020 ps/nm every 60 km. Therefore, a dispersion compensating (DC) module containing a DC fiber has to be inserted into the system at every amplifier stage that accounts for about 1000 ps/nm accumulated dispersion. As this length of DC fiber does not account for any real transmission distance, it is desirable to keep this length as short as possible. This implies that the negative dispersion of the DC fiber must be maximized. However, as the dispersion is made more negative via increasing the role played by waveguide dispersion, the fiber becomes more bend sensitive and the base attenuation of the fiber increases. Therefore, most value is gained by maximizing dispersion (D) while simultaneously keeping attenuation (Attn) as low as possible. Thus, the ratio of |D/Attn|, known as the figure of merit, must be maximized rather than dispersion alone.
Until recently, system and DC fiber designers had considered only one channel (1550 nm). That is, a DC fiber would be used to compensate dispersion at only one wavelength, and hence the dispersion slope of the fiber was not important. However, with the new emphasis on WDM technology, it has become necessary to provide dispersion compensation over all wavelengths of transmission within the erbium fiber amplifier window. This implies that designers are now restricted by the channel that has the worst compensation. An obvious solution to the above quandary is to design a DC fiber such that dispersion is simultaneously compensated at all wavelengths. Thus, there is an added criterion to satisfy, namely, dispersion slope. The figure of merit must be maintained at a large value for all wavelengths at which the DC fiber is to be utilized. As the bend-edge causes increased attenuation at longer wavelengths, DC fibers that have a low bend edge have been limited to use at C-band wavelengths (up to 1565 nm) that are substantially unaffected by this effect.
To examine the effect of dispersion slope on the system assume that a system employs the aforementioned SMF fiber, which has a dispersion of +17 ps/nm/km and dispersion slope of about +0.056 ps/nm2xc2x7km at 1550 nm. Consider the effect of five different DC fibers on the system. The dispersion and dispersion slope characteristics of the five fibers are shown in Table 1, wherein dispersion, D is expressed in units of ps/nmxc2x7km, and dispersion slope, Dslope is expressed in units of ps/nm2xc2x7km.
DC fibers having dispersions of xe2x88x9285 and xe2x88x92102 ps/nmxc2x7km have been chosen for this theoretical example since a length L of DC fiber having a dispersion of xe2x88x9285 ps/nmxc2x7km will compensate for a length 5L of SMF fiber having a dispersion of 17 ps/nmxc2x7km, and a length L of DC fiber having a dispersion of xe2x88x92102 ps/nm km will compensate for a length 6L of that SMF fiber.
Using the characteristics of the SMF fiber and the DC fiber, the uncompensated dispersion at the end channels (1530 nm and 1565 nm) of the erbium C band window can be calculated, assuming that all DC fibers are designed for complete compensation at 1550 nm. Calculated values are given in columns 4 and 5 of Table 1. If it is assumed that the system is pulse spectral width limited, then the relationship between dispersion, bit rate and total length is given by equation 1,
B(|xcex22|L)xc2xd less than xc2xcxe2x80x83xe2x80x83(1)
where B is the bit rate, xcex22=(Dxcex2)/2xcfx80c, and L is the length.
Equation 1 can be rewritten in terms of bit rate and the total dispersion accumulated in a given length. Based on the above relationship, given a bit rate and the average accumulated dispersion, one can determine the total length of a system before dispersion becomes a limiting factor, and this length is given for bit rates of 10 and 40 Gbs in columns 6 and 7 of Table 1. DC fibers 1, 2, 3 and 4 are theoretical examples which are used herein to demonstrate the effects of various dispersions and dispersion slopes on system length. DC fiber 5 is a commercial fiber that compensates for dispersion at only one wavelength, eg. 1550 nm. Dispersion slope is not listed for DC fiber 5 since dispersion slope was not specified for DC fibers intended for operation at a single wavelength, and dispersion slope could vary between approximately xe2x88x920.5 and +0.5 ps/nm2xc2x7km without adversely affecting system operation. It is noted that DC fibers 1, 2, 3 and 4 are suitable for use in a 10 Gbs system in that their use in such a system enables signal transmission over a distance of at least 600 km. Of the five listed fibers only DC fiber 3 is suitable for use in a 40 Gbs system.
The xcexa value of a DC fiber is defined herein as
xcexa=(DDC)/(DSlopeDC)xe2x80x83xe2x80x83(2)
where DDC and DslopeDC are the dispersion and dispersion slope of the DC fiber. Relative dispersion slope (RDS), the reciprocal of xcexa, is sometimes used to characterize a ratio of dispersion and dispersion slope. The ratio of the dispersion to dispersion slope of the SMF fiber is about 303. DC Fiber 3 is unique, since the dispersion and the dispersion slope of that DC fiber are such that essentially complete compensation can be achieved over all wavelengths. In other words, the xcexa value of DC fiber 3 is also 303. This criterion is defined as full compensation. Line 20 of FIG. 2 is referred to as the line of full compensation, as its slope is 303. DC fiber 3 is represented by that point on line 20 where dispersion is xe2x88x9285 ps/nmxc2x7km and dispersion slope is xe2x88x920.28 ps/nm2xc2x7km. Other fibers falling on line 20, such as one having a dispersion of xe2x88x92102 ps/nmxc2x7km and a dispersion slope of xe2x88x920.336 ps/nm2xc2x7km, for example, would also afford full compensation.
Although DC fiber 3 is superior to DC fibers 1, 2 and 4 for a 10 Gbs system, it does not add value, as terrestrial systems are designed primarily for a maximum distance of about 600 km. Thus, certain DC fibers which do not provide complete compensation are suitable for use in DC modules, and, if these DC fibers are more easily produced than those that do provide complete compensation, they would be preferred. DC fibers 1, 2 and 4 have xcexa values of 457, 548 and 380, respectively. Values of xcexa that are lower than those of DC fibers 1, 2 and 4 and which are closer in value to 303 correspond to enhanced dispersion properties and thus to longer transmission distances. DC fibers 4 and 1 are arbitrarily selected for presentation in FIG. 2 where they are represented by lines 21 and 22, respectively. Shaded region 23 between lines 21 and 22 represents one group of DC fibers that provide acceptable dispersion and dispersion slope, and moreover, they can be employed in 10 Gbs WDM systems longer than 1000 km. DC fibers having dispersion properties between lines 20 and 21 and even those having properties falling below line 20 and near thereto would be suitable for use in DC modules, but it is not necessary to use fibers having such low xcexa values in 10 Gbs systems.
Only those DC fibers that have dispersion properties on or very near line 20 would be suitable for use in 40 Gbs systems.
Erbium fiber amplifiers that are presently being developed will operate in the L-band, which includes wavelengths longer than the current limit of 1565 nm. Presently available DC fibers, which provide acceptable dispersion properties, are not suitable for use at such longer wavelengths as their bend-edge wavelength is sufficiently low that loss is unacceptable at wavelengths longer than 1565 nm. The bend-edge is moved to shorter wavelengths as the cutoff wavelength (xcexCO) decreases and as the mode field diameter (MFD) increases. In either case the light is very weakly guided. Therefore, in order to push the bend-edge to higher wavelengths, xcexCO must be increased, and/or MFD must be decreased.
If the MFD is too small, non linear effects such as cross phase modulation and self phase modulation increase and splice loss increases. Therefore, MFD should be greater than 4 xcexcm and preferably greater than 4.5 xcexcm.
In order to provide desired low values of negative dispersion and negative dispersion slope, prior DC fiber designs have resulted in cutoff wavelengths below about 1000 nm, some being below 800 nm. For bend-edge wavelength to exceed 1565 nm in a DC fiber exhibiting desirable dispersion properties, cutoff wavelength should be higher than 1000 nm, and is preferably higher than 1300 nm. With present DC fiber designs, it is very difficult to have a cutoff wavelength longer than 1000 nm in fibers exhibiting a dispersion slope more negative than xe2x88x920.2 ps/nm2xc2x7km. Cutoff wavelength should be sufficiently shorter than the lowest operating wavelength, preferably about 40 nmxe2x88x9250 nm shorter, to avoid an increase in attenuation. For a system operating in the erbium amplifier band, xcexCO should be shorter than about 1500 nm.
Prior Dispersion Compensating Fiber Designs
As the large positive dispersion accumulated by transmission at 1550 nm over 1300 nm zero D fibers has been unacceptable for long distance signal transmission, dispersion compensating optical fibers have been employed in such 1550 nm systems. These dispersion compensation fibers exhibit large negative dispersion and may also exhibit negative dispersion slope. FIGS. 1A and 1B show index profiles of two types of previously employed DC fibers that provide suitable values of dispersion (Dxe2x89xa6xe2x88x9280 ps/nmxc2x7km) and dispersion slope (DSlopexe2x89xa6xe2x88x920.15 ps/nm2xc2x7km) and exhibit a bend-edge wavelength suitable for C-band systems that operate at wavelengths up to 1565 nm. More negative values of dispersion and dispersion slope can be achieved in such fibers; however, other characteristics such as bending loss are adversely affected.
The W-type three-layer index profile of FIG. 1A includes a central core 1, a second core layer or moat 2 and cladding 3. Fibers of the type represented by FIG. 1A are disclosed in U.S. Pat. No. 5,361,319. The diameters of core 1 and moat 2 are a and b, respectively. The normalized refractive indices of core 1 and moat 2 with respect to cladding 3 are xcex94+ and xcex94xe2x88x92, respectively. The xcex94 of a core layer having a refractive index nx is given by (nx2xe2x88x92nCL2)/2nx2, where nCL is the refractive index of the cladding. The ratio a/b as well as the previously mentioned core characteristics can be optimized to achieve large negative values of dispersion accompanied by negative disperson slope. It has been recognized that designs that optimize negative dispersion and negative dispersion slope can suffer from bending loss, light propagation problems and the like. The cutoff wavelengths of these W-type fibers are below 1000 nm, and bend-edge wavelengths are lower than 1700 nm.
FIG. 1B shows the index profile of another type of DC fiber that includes an additional core feature, viz. a positive delta ring immediately adjacent the moat, for modifying light propagation charactistics. Fibers that include a ring adjacent the moat region of the core are also disclosed in U.S. Pat. No. 5,361,319.
The fiber of FIG. 1B includes a central core 11 that is surrounded by moat region 12 which is in turn surrounded by ring 13. The normalized refractive indices of central core 11, moat 12 and ring 13 with respect to cladding 3 are xcex94C, xcex94M and xcex94R, respectively, where xcex94C equals (nC2xe2x88x92nCL2)/2nC2, xcex94M equals -(nM2xe2x88x92nCL2)/2nM2 and xcex94R equals (nR2xe2x88x92nCL2)/2nR2, where nC, nR, and nCL are the peak refractive indices of the central core region, ring, and cladding, respectively, and nM is the minimum refractive index of the moat. The outer radii of the central core 11, moat 12 and ring 13 are rC, rM and rR, respectively.
Curve 24 of FIG. 2 is a plot of dispersion vs. dispersion slope for a particular type FIG. 1B fiber profile. The fiber characteristics for a negative dispersion of xe2x88x9280 ps/nmxc2x7km were: xcex94C=xcx9c1.9%, xcex94M=xe2x88x920.52%, xcex94R=0.25%, rC=1.65 xcexcm, rM=3.6 xcexcm, and rR=3.95 xcexcm. Each data point in the graph represents the optical properties for a given core radius of the profile of FIG. 1B. Fibers having different outside diameters, and thus different core radii, were drawn, and the refractive index profiles were measured and were input to a computer model that generated the fiber dispersion characteristics. Different core diameters are obtained for a given profile by initially forming a plurality of identical DC fiber core preforms; each preform is provided with a different overclad thickness. When the resultant draw blanks are drawn to predetermined outside diameters, the core radii are different. The data point at the far right of the curve (where dispersion is aboutxe2x80x9442 ps/nmxc2x7km) represents the largest diameter; core diameter decreases at data points located to the left where dispersion is more negative. It has been preferred to employ curve 24-type fibers having dispersions around xe2x88x9285 ps/nmxc2x7km as they are less bend sensitive than those exhibiting more negative dispersions.
As previously indicated, 600 km 10 Gbs C-band systems do not need a full compensation solution (represented by line 20 of FIG. 2). The adequate solution falling within shaded area 23 of FIG. 2 meets the requirements of a 10 Gbs system while providing some margin of error. Some presently available DC fibers meet these requirements. Moreover, some presently available DC fibers have characteristics that fall on or near line 20 of FIG. 2 whereby they would be suitable for use in 40 Gbs C-band systems operating at wavelengths less than 1560 nm.
Sensitivity analysis and optical space mapping were performed on the present day DC profile shown in FIG. 1B to provide desired dispersion properties while improving other fiber characteristics. Only the essential results of that analysis are given. As the xcexa values of these analyzed profiles were greater than 303, a decrease in K represents an improvement in dispersion characeristics.
The DC fiber of the present invention provides the necessary negative dispersion and negative dispersion slope required for compensating dispersion in WDM systems operating at 10 or more Gbs, and preferably with the capability of operating in the L-band.
One aspect of the invention is a dispersion compensating optical fiber the profile of which is such that cutoff wavelength is sufficiently long to enable the use of the fiber in the L-band while maintaining desirable values of dispersion and dispersion slope. Another aspect is a dispersion compensating optical fiber having negative dispersion and dispersion slope properties suitable for use in SMF based WDM systems operating at bit rates of at least 10 Gbs. Yet another aspect is a dispersion compensating optical fiber that exhibits large values of both negative dispersion and negative dispersion slope and yet is not hampered by bend sensitivity. Another aspect of the invention is an optical transmission system including at least 40 km of single-mode optical fiber optimized for low dispersion operation at 1290-1330 nm in series with a much shorter length of dispersion compensating optical fiber having negative dispersion and dispersion slope properties suitable for use in WDM systems operating at bit rates of at least 10 Gbs at wavelengths greater than 1520 nm, and preferably at wavelengths greater than 1570 nm. A further aspect is a method of making a dispersion compensating optical fiber such that a region containing a readily diffusing dopant can be situated in close proximity to an undoped region.
One embodiment of the invention relates to a dispersion compensating optical fiber core of transparent material surrounded by a cladding layer of transparent material of refractive index nCL. The core includes a central core region having a maximum refractive index nC such that xcex94C is greater than +1.2% surrounded by a moat region having a minimum refractive index nM such that xcex94Mxe2x89xa6xe2x88x920.4%, which is surrounded by a ring region that includes a segment where refractive index increases with increasing radius to a refractive index of at least nR such that xcex94Rxe2x89xa6+0.15%. The segment is located at a radius that is at least 0.3 xcexcm beyond the moat region. The refractive index profile of the fiber is such that the dispersion slope of the fiber is more negative than xe2x88x920.15 ps/nm2xc2x7km at a wavelength of about 1550 nm. The dispersion slope of the fiber is preferably more negative than xe2x88x920.2 ps/nm2xc2x7km at a wavelength of about 1550 nm, while the dispersion at that wavelength is preferably more negative than xe2x88x9280 ps/nmxc2x7km.
The ring region can include inner and outer portions having maximum refractive indices nR1 and nR2, respectively, that are greater than nCL. The inner and outer ring portions can be separated by an inter-ring region having a refractive index nS that is less than nR1 and nR2, or the inner ring portion can be situated immediately adjacent the outer ring portion. The index profile can be such that xcex94R1 can be equal to zero, less than zero or greater than zero.
The outer portion of the ring region can include a peak having a maximum refractive index n2 such that xcex94R2xe2x89xa7+0.15%. The maximum refractive index of the peak can be located between 0.3 xcexcm and 3 xcexcm from the outer edge of moat region, and is preferably located between 1 xcexcm and 2.5 xcexcm from the outer edge of moat region.
The cladding layer of the fiber can consist of silica doped with a refractive index increasing dopant, and the moat region can be formed of silica doped with a refractive index decreasing dopant such as, but not limited to, fluorine.
In accordance with a further embodiment, a dispersion compensating optical fiber includes a core of transparent material surrounded by a cladding layer of transparent material having a refractive index nCL. The core includes three adjacent regions named in order of increasing radius: (a) a central core region having a maximum refractive index nC, (b) a moat having a minimum refractive index nM, and (c) a ring region including distinctive inner and outer portions having maximum refractive indices nR1 and nR2, wherein nC greater than nR1 greater than nCL greater than nM and nC greater than nR2 greater than nCL greater than nM. The radial refractive index plot of the fiber is characterized in that the area under the outer half of the ring region is greater than the area under the inner half of the ring region. The central core region of the fiber preferably has a maximum refractive index nC such that xcex94C is greater than +1.2%, and the moat region preferably has a minimum refractive index nM such that xcex94Mxe2x89xa6xe2x88x920.4%.
Yet another embodiment of the invention relates to a dispersion compensating optical fiber including a core of transparent material surrounded by a cladding layer of transparent material having a refractive index nCL. The core has a central core region having a maximum refractive index nC such that xcex94C is greater than +1.2% surrounded by a moat region having a minimum refractive index nM such that xcex94Mxe2x89xa6xe2x88x920.4%. The moat region is surrounded by a ring region including distinctive inner and outer portions having maximum refractive indices nR1 and nR2, respectively, that are greater than nCL, whereby xcex94R1 and xcex94R2 are positive. The refractive index profile of the fiber is such that the cutoff wavelength is greater than 1000 nm and the dispersion slope is more negative than xe2x88x920.2 ps/nm2xc2x7km at a wavelength of about 1550 nm.
Another embodiment concerns a dispersion compensated optical transmission link including the serial combination of at least 40 km of standard single-mode transmission fiber optimized for low dispersion operation at a wavelength in the range between 1290 and 1330 nm, and a dispersion compensating optical fiber that includes a core of transparent material surrounded by a cladding layer of transparent material having a refractive index nCL. The core includes a central core region having a maximum refractive index nC such that xcex94C is greater than +1.2%, surrounded by a moat region having a minimum refractive index nM such that xcex94Mxe2x89xa6xe2x88x920.4%. The moat region is surrounded by a ring region that includes a segment where refractive index increases with increasing radius to a refractive index of at least nR such that xcex94Rxe2x89xa7+0.15%. The segment is located at a radius that is at least 0.3 xcexcm beyond the moat region. The refractive index profile of the fiber is such that the dispersion slope of the fiber is more negative than xe2x88x920.15 ps/nm2xc2x7km at a wavelength of about 1550 nm, whereby the system is capable of operating at bit rates of at least 10 Gbs at wavelengths greater than 1520 nm.
The invention also concerns a method of forming an optical device. A first coating of base glass particles is deposited on a mandrel, and a second coating of glass particles is deposited on the outer surface of the first coating, the second coating being formed of the base glass and at least one dopant. The mandrel is removed from the resultant porous glass preform to form a longitudinal aperture through the preform. The porous preform is dried and sintered to form a solid glass tube having an inner region that is substantially free of the at least one dopant. The step of depositing the first coating includes depositing a first layer of the base glass particles on the mandrel at a first density, depositing a transition layer of the base glass particles on the first layer such that the density of the transition layer varies from the first density at the first layer to a second density at the outer surface of the transition layer, the second density being at least 30 percent less than the first density, and depositing a third layer of the base glass particles on the transition layer at the second density.
The step of depositing a first coating can include feeding at a first rate a first reactant to a burner to generate in the flame a stream of the base glass particles. The stream is directed onto the mandrel to deposit the first layer. The flow rate of the first reactant is gradually increased from the first flow rate to a second flow rate to form the transition layer. The first reactant is fed to the burner at the second rate to form the third layer. The first flow rate is preferably less than 70% of the second flow rate. Moreover, the flame is preferably hotter during the deposition of the first layer than it is during the deposition of the second layer.
In an embodiment wherein the base glass is SiO2 and the dopant is GeO2, and wherein the first reactant is SiCl4, the flow rate of the SiCl4 during the formation of the first layer is preferably less than 70% of the flow rate of SiCl4 during the formation of the third layer. Moreover, the tendency for GeO2 to diffuse and deposit in the inner region of the glass tube is reduced by flowing at least 75 sccm chlorine into the longitudinal aperture during the sintering step.
Yet another aspect of the present invention relates to a method of forming a fluorine containing glass article. The method includes the steps of forming a porous, fluorine containing glass preform, and heating the formed preform to a first temperature to sinter the preform. The sintered preform is then exposed to a temperature of at least 1000xc2x0 C. and lower than the first temperature.
It is to be understood that both the foregoing general description and the following detailed description are merely exemplary of the invention, and are intended to provide an overview or framework for understanding the nature and character of the invention as it is claimed. The accompanying drawings are included to provide a further understanding of the invention, and are incorporated in and constitute a part of this specification. The drawings illustrate various embodiments of the invention, and together with the description serve to explain the principles and operation of the invention.
FIGS. 1A and 1B are refractive index profiles of two common types of prior art dispersion compensating optical fibers.
FIG. 2 is a plot showing the relationship between dispersion and dispersion slope for different values of fiber outside diameter for the fiber of FIG. 1B, and it additionally shows acceptable regions for dispersion and dispersion slope for DC fibers suitable for use in high bit rate systems.
FIG. 3 is an idealistic refractive index profile of one aspect of the dispersion compensating fiber of this invention.
FIG. 4 is a refractive index profile of one embodiment of the dispersion compensating fiber of the present invention.
FIGS. 5A, 5B and 5C are refractive index profiles of further embodiments of the present invention.
FIG. 6 shows a plurality of index profiles depicting updoping of the cladding.
FIG. 7 is a plot showing the relationship between dispersion and dispersion slope for the fiber profiles characterized in Table 4.
FIG. 8 is a plot showing the relationship between the ratio of dispersion to dispersion slope of the various DC fiber profiles characterized in Table 4 as a function of dispersion.
FIG. 9 schematically depicts an optical transmission system employing a dispersion compensating fiber.
FIG. 10 schematically shows the deposition of glass particles to form a porous glass preform used in the manufacture of a dispersion compensating fiber in accordance with the present invention.
FIG. 11 is a cross-sectional view illustrating the consolidation of a porous preform onto a glass rod to form a preform used in the manufacture of a dispersion compensating fiber.